737 Background: A is approved in combination with FOLFIRI (A-FOLFIRI) for the treatment of mCRC patients (pts) previously treated with oxaliplatin. PULSAR trial (NCT02173990) aims to measure the activity A-FOLFIRI as 1st-line mCRC treatment. Key secondary objectives are safety, as well as circulating biomarkers and dynamic contrast-enhanced ultrasound measurements as PFS prognosticators. We present here interim analysis of safety and activity. Methods: Pts received A (4 mg/kg I.V.) every 2 weeks in combination with FOLFIRI (Irinotecan: 180 mg/m2, leucovorin: 200 mg/m2, Bolus FU: 400 mg/m2, infusional FU: 2400 mg/m2 over 46hrs). The primary endpoint is 10-month [m] PFS (RECIST 1.1). The target sample size is 72 pts for the following assumptions: P0 = 0.40, P1 = 0.55, α = β = 0.1. Results: Among the first 30 recruited mCRC pts with unresectable metastases (mets), 29 received at least 1 cycle of A-FOLFIRI. 30, 23, and 28 pts were evaluable for survival (intent-to-treat), tumor response, and safety, respectively. Median age was 61.5 (45-82), 70% pts were men. ECOG PS was 0, 1, and 2, in 47%, 43%, and 10% of the pts. 6/30 pts received previous adjuvant chemotherapy. 22/30 pts presented with synchronous mets. 26/30 pts presented with liver mets. Median treatment duration was 6.3m (0.5-24.3) and pts (n = 29) received a median number of 10 cycles (1-37) of A and of 11 cycles (1-43) of FOLFIRI. Median follow-up was 7.2m (1.6-23.9). The 10-m PFS was 59.9% [95%-CI: 37.5-76.5]. Median PFS was 12.9m (95% CI: 6.1-…). 12-m OS rate was 73.7% (95% CI: 48.3-87.9). 5 pts died from PD, and 1 from treatment-related toxicity (colon perforation). 27/28 pts presented at least one severe Adverse Event (AE). Most frequent severe AE (related or not) were hypertension (43%), neutropenia (29%), diarrhea (18%), alkaline phosphatase increase (18%), fatigue (18%), GGT increase (14%), and weight loss (14%). Conclusions: A-FOLFIRI seems promising in 1st line setting mCRC, with respect to PFS. Due to high rates of severe AEs, dose modifications have to be proceeded in a coming protocol amendment. Funded by Sanofi. Clinical trial information: NCT02173990.